1. Field of the Invention
This invention relates to a method for making a retarder, more particularly to a method for making a retarder that has at least one thick layer region and at least one thin layer region.
2. Description of the Related Art
A liquid crystal has different refractive indices indifferent axes (i.e., birefringence). In many optical applications, the birefringence of the liquid crystal is commonly utilized for forming a retarder with a desired phase difference value. As such, a backlight module having the retarder can be used to improve a contrast and a viewing angle of a display. Moreover, when a 3D display device includes a display panel attached with a retarder having alternating first and second regions in a plane direction of the retarder to provide a phase difference value, it can provide a 3D image. The 3D display device may be a naked eye type display or an eyeglass type display.
Japanese Patent Publication No. 2001-100150 discloses a mechanical method for forming a retarder. In this publication, after a retarder film is attached to a hard substrate, an undesired portion of the retarder film is cut using a cutter so as to obtain a retarder having two different regions to provide a phase difference of λ/2. However, the mechanical method is relatively complex, and the cutter is likely to deform after repeating the cutting process, thereby reducing the yield rate of the retarder. Moreover, waste chips produced during the cutting process may also be a factor of the reduced yield rate. Therefore, this method is not suitable for mass production.
In order to settle the above problems, a method for making a retarder, which is disclosed in US patent application publication no. 2007/0020404, has been proposed. In this method, a mixture of a cross-linkable liquid crystal polymer and a non-cross-linkable liquid crystal polymer is applied to an alignment layer formed on a substrate, thereby forming a liquid crystal layer on the alignment layer. Then, the liquid crystal layer is exposed to UV light so as to induce a cross-linking reaction of the cross-linkable liquid crystal polymer. Thereafter, the non-cross-linkable liquid crystal polymer in the liquid crystal layer is removed using a solvent. Accordingly, the liquid crystal layer has a required phase structure, such as a microstructure having indentations or grooves, thus obtaining a phase difference distribution.
However, this method is still complex, and the phase structure formed by the method is less likely to have a regular arranged pattern. Thus, if the retarder is not precisely positioned in a 3D display, double image or the so-called “ghost” may occur.
In addition, U.S. Pat. No. 5,926,241 discloses a method to make retarders. In this method, a liquid crystal layer is formed on a substrate, and heated until the liquid crystal layer exhibits an isotropic phase (at this time, the liquid crystals still have no phase difference). Then, the liquid crystal layer is exposed to UV light through a patterned mask to cure specific regions of the liquid crystal layer. Thereafter, the liquid crystal layer is cooled to room temperature, thereby aligning the liquid crystals in uncured regions of the liquid crystal layer. At this time, the liquid crystal layer has two different regions, exhibits an anisotropic phase, and has a phase difference of λ/2. Finally, the liquid crystal layer is fully exposed to UV light and fully cured, thereby forming a retarder.
However, at an interface between the two different regions (i.e., the region cured at a relatively high temperature and the region cured at room temperature), the liquid crystals may align in an irregular fashion, resulting in light leakage of a display and poor display quality.